• 화약ㆍ발파
• /
• 제23권1호
• /
• pp.47-54
• /
• 2005

본 연구에서는 $PFC^{3D}$를 사용하여 시멘트 모르타르와 굵은 골재로 이루어진 콘크리트 기둥의 발파과정에서 나타나는 폭발과 파괴현상을 모사하여 보았다. 폭원모델링 과정에서는 공내입자들의 반경을 팽창/수축시키는 기법을 통해 공벽입자들에 접촉력의 형태로 폭발압력을 부여하는 방법을 사용하였다. 현장 발파실험에서는 철근콘크리트 기둥을 대상으로 초안폭약을 사용하여 발파하고 그 파괴거동을 고속카메라를 이용하여 관찰하였다. 모사과정에서는 철근의 규격과 입자요소의 크기에 따른 해석시간을 고려하여 모르타르와 굵은 골재로 이루어진 콘크리트 기둥을 대상으로 제안된 폭원모델링 기법을 적용하여 해석을 실시하였다. 해석결과 나타난 저항선의 이동속도는 $17\~24\;m/s$로서 실험치 $14\~18\; m/s$를 약간 상회하고 있으나 제안된 폭원모델링 기법을 사용한다면 암석이나 기타 재료들에 대한 발파과정에서 나타는 파괴거동을 수치적으로 보다 유사하게 모사할 수 있을 것으로 판단된다.

• 화약ㆍ발파
• /
• 제22권4호
• /
• pp.7-15
• /
• 2004

본 연구에서는 PFC3D를 사용한 폭원모델링 기법을 제안하고, 제안된 기법을 시멘트 모르타르와 같은 연약재료의 발파에 적용하여 그 적용성을 시험해 보았다. PFC3D는 개별요소법(DEM)을 기반으로 하고 있어 응력파의 전파와 재료의 동적 파괴현상을 모사하는데 적합한 코드로 분류된다. 폭원모델링 과정에서는 공내입자들의 반경을 팽창/수축시키는 기법을 통해 공벽입자들에 접촉력의 형태로 폭발압력을 부여하는 방법을 사용하였으며, 입력하중에 따라 공벽에서 유발되는 접촉력을 계산단계마다 측정 및 보정함으로써 폭발압력의 크기를 제어할 수 있도록 하였다. 시멘트 모르타르 블록의 발파모델링 과정에서는 기존의 외력을 이용하는 방법과 본 연구에서 제안하고 있는 접촉력을 이용하는 기법을 각기 적용함으로써 연약재료의 파괴과정을 정성적으로 비교하여 보았다. 해석결과, 제안된 폭원모델링 기법을 적용한다면 암석이나 콘크리트와 같은 공학재료들이 발파과정에서 보이는 파괴거동을 수치적으로 보다 유사하게 모사 할 수 있을 것으로 판단된다.

• Geomechanics and Engineering
• /
• 제22권5호
• /
• pp.385-396
• /
• 2020

A uniaxial compression test was performed to analyse the mechanical properties and macroscale and mesoscale failure mechanisms of sandstone with pyrite concretions. The effect of the pyrite concretions on the evolution of macroscale cracks in the sandstone was further investigated through numerical simulations with Particle Flow Code in 2D (PFC2D). The results revealed that pyrite concretions substantially influence the mechanical properties and macroscale and mesoscale failure characteristics of sandstone. During the initial loading stage, significant stress concentrations occurred around the edges of the pyrite concretion accompanied by the preferential generation of cracks. Meanwhile, the events and cumulative energy counts of the acoustic emission (AE) signal increased rapidly because of friction sliding between the concretion and sandstone matrix. As the axial stress increased, the degree of the stress concentration remained relatively unchanged around the edges of the concretions. The cracks continued growing rapidly around the edges of the concretions and gradually expanded toward the centre of the sample. During this stage, the AE events and cumulative energy counts increased quite slowly. As the axial stress approached the peak strength of the sandstone, the cracks that developed around the edges of the concretion started to merge with cracks that propagated at the top-left and bottom-right corners of the sample. This crack evolution ultimately resulted in the shear failure of the sandstone sample around the edges of the pyrite concretions.

• Steel and Composite Structures
• /
• 제10권2호
• /
• pp.129-149
• /
• 2010

The objective of this study is to formulate a general 3D material-structural analysis framework for the thermomechanical behavior of steel-concrete structures in a fire environment. The proposed analysis framework consists of three sequential modeling parts: fire dynamics simulation, heat transfer analysis, and a thermomechanical stress analysis of the structure. The first modeling part consists of applying the NIST (National Institute of Standards and Technology) Fire Dynamics Simulator (FDS) where coupled CFD (Computational Fluid Dynamics) with thermodynamics are combined to realistically model the fire progression within the steel-concrete structure. The goal is to generate the spatial-temporal (ST) solution variables (temperature, heat flux) on the surfaces of the structure. The FDS-ST solutions are generated in a discrete form. Continuous FDS-ST approximations are then developed to represent the temperature or heat-flux at any given time or point within the structure. An extensive numerical study is carried out to examine the best ST approximation functions that strike a balance between accuracy and simplicity. The second modeling part consists of a finite-element (FE) transient heat analysis of the structure using the continuous FDS-ST surface variables as prescribed thermal boundary conditions. The third modeling part is a thermomechanical FE structural analysis using both nonlinear material and geometry. The temperature history from the second modeling part is used at all nodal points. The ABAQUS (2003) FE code is used with external user subroutines for the second and third simulation parts in order to describe the specific heat temperature nonlinear dependency that drastically affects the transient thermal solution especially for concrete materials. User subroutines are also developed to apply the continuous FDS-ST surface nodal boundary conditions in the transient heat FE analysis. The proposed modeling framework is applied to predict the temperature and deflection of the well-documented third Cardington fire test.

• Structural Engineering and Mechanics
• /
• 제81권1호
• /
• pp.81-91
• /
• 2022

The effects of interaction between concrete-gypsum interface and edge crack on the failure behavior of the specimens in senicircular bend (SCB) test were studied in the laboratory and also simulated numerically using the discrete element method. Some quarter circular specimens of gypsum and concrete with 5 cm radii and hieghts were separately prepared. Then the semicircular testing specimens were made by attaching one gypsum and one concrete sample to one another using a special glue and one edge crack is produced (in the interface) by do not using the glue in that part of the interface. The tensile strengths of concrete and gypsum samples were separately measured as 2.2 MPa and 1.3 MPa, respectively. during all testing performances a constant loading rate of 0.005 mm/s were stablished. The proposed testing method showed that the mechanism of failure and fracture in the brittle materials were mostly governed by the dimensions and number of discontinuities. The fracture toughnesses of the SCB samples were related to the fracture patterns during the failure processes of these specimens. The tensile behaviour of edge notch was related to the number of induced tensile cracks which were increased by decreasing the joint length. The fracture toughness of samples was constant by increasing the joint length. The failure process and fracture pattern in the notched semi-circular bending specimens were similar for both methods used in this study (i.e., the laboratory tests and the simulation procedure using the particle flow code (PFC2D)).

• Geomechanics and Engineering
• /
• 제28권2호
• /
• pp.117-133
• /
• 2022

Determination of the mechanical behaviour of jointed rock masses has been a challenge for rock engineers for decades. This problem is more pronounced for non-persistent jointed rock masses due to complicated interaction of rock bridges on the overall behaviour. This paper aims to study the effect of a non-persistent joint set configuration on the mechanical behaviour of rock materials under both uniaxial and biaxial compression tests using a discrete element code. The numerical simulation of biaxial compressive strength of rock masses has been challenging in the past due to shortcomings of bonded particle models in reproducing the failure envelope of rock materials. This problem was resolved in this study by employing the flat-joint contact model. The validity of the numerical model was investigated through a comprehensive comparative study against physical uniaxial and biaxial compression experiments. Good agreement was found between numerical and experimental tests in terms of the recorded peak strength and the failure mode in both loading conditions. Studies on the effect of joint orientation on the failure mode showed that four zones of intact, transition to block rotation, block rotation and transition to intact failure occurs when the joint dip angle varies from 0° to 90°. It was found that the applied confining stress can significantly alter the range of these zones. It was observed that the minimum strength occurs at the joint dip angle of around 45 degrees under different confining stresses. It was also found that the joint orientation can alter the post peak behaviour and the lowest brittleness was observed at the block rotation zone.

• Advances in concrete construction
• /
• 제13권 2호
• /
• pp.199-213
• /
• 2022

Experimental and discrete element approaches were used to examine the effects of F shape non-persistent joints on the failure behaviour of concrete under uniaxial compressive test. concrete specimens with dimensions of 200 cm×200 cm×50 cm were provided. Within the specimen, F shape non-persistent joint consisting three joints were provided. The large joint length was 6 cm, and the length of two small joints were 2 cm. Vertical distance between two small joints change from 1.5 cm to 4.5 cm with increment of 1.5 cm. In constant joint lengths, the angle of large joint change from 0° to 90° with increments of 30°. Totally 12 different models were tested under compression test. The axial load rate on the model was 0.05 mm/min. Concurrent with experimental tests, numerical simulation (Particle flow code in two dimension) were performed on the models containing F shape non-persistent joint. Distance between small joints and joint angles were similar to experimental one. the results indicated that the failure process was mostly governed by both of the Distance between small joints and joint angles. The axial loading rate on the model was 0.05 mm/min. The compressive strengths of the samples were related to the fracture pattern and failure mechanism of the discontinuities. Furthermore, it was shown that the compressive behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the joint angle. In the first, there were only a few acoustic emission (AE) hits in the initial stage of loading, and then AE hits rapidly grow before the applied stress reached its peak. Furthermore, a large number of AE hits accompanied every stress drop. Finally, the failure pattern and failure strength are similar in both approaches i.e., the experimental testing and the numerical simulation approaches.

• 터널과지하공간
• /
• 제23권6호
• /
• pp.493-505
• /
• 2013

본 수치해석논문에서는 절리와 단층대를 포함한 지열저류층에 수리자극을 가할 시 수반되는 유도지진과 단층대의 변형을 개별요소법을 사용하여 모델링하였다. 수채해석기법은 2차원 입자유동코드를 기반으로 하며 수리역학적 상호작용기법과 미소파괴음의 모멘트텐서 역산알고리즘이 결합되었다. 수치해석의 주요결과로는 시공간적으로 변하는 유도지진의 분포와 규모 그리고 단층대의 변형(파괴 및 전단변위)과 주입유체압력의 시공간적 분포와의 상관관계이다. 첫 번째 수치해석으로부터 절리가 분포하는 지열저류층에서의 수리자극에 의한 유도지진의 분포는 주입유체의 점성에 상당한 영향을 받는 것으로 나타났다. 주입유체의 점성이 낮은 경우 (1 cP), 유도지진의 발생범위가 큰 것으로 나타났으며, 주입 후 발생하는 유도지진의 개수와 규모 또한 높게 나타났다. 단층대가 존재하는 지열저류층의 수리자극 모델링의 결과, 주입정의 위치가 단층대와 가까운 경우 작은 주입수 압력분포(<0.1 MPa)로도 단층대의 파괴와 전단변형을 일으킬 수 있는 것으로 나타났다. 본 논문에서 소개한 수치해석기법은 수리자극을 통한 지열저류층 개발 시 유도지진의 분포와 규모를 실제 유체주입작업전에 예측할 수 있게 함으로써 지열에너지개발 분야에서 유용하게 사용될 수 있을 것으로 기대한다.

• Structural Engineering and Mechanics
• /
• 제86권5호
• /
• pp.607-619
• /
• 2023

This study, it was tried to evaluate the asphalt behavior under tensile loading conditions through indirect Brazilian and direct tensile tests, experimentally and numerically. This paper is important from two points of view. The first one, a new test method was developed for the determination of the direct tensile strength of asphalt and its difference was obtained from the indirect test method. The second one, the effects of particle size and loading rate have been cleared on the tensile fracture mechanism. The experimental direct tensile strength of the asphalt specimens was measured in the laboratory using the compression-to-tensile load converting (CTLC) device. Some special types of asphalt specimens were prepared in the form of slabs with a central hole. The CTLC device is then equipped with this specimen and placed in the universal testing machine. Then, the direct tensile strength of asphalt specimens with different sizes of ingredients can be measured at different loading rates in the laboratory. The particle flow code (PFC) was used to numerically simulate the direct tensile strength test of asphalt samples. This numerical modeling technique is based on the versatile discrete element method (DEM). Three different particle diameters were chosen and were tested under three different loading rates. The results show that when the loading rate was 0.016 mm/sec, two tensile cracks were initiated from the left and right of the hole and propagated perpendicular to the loading axis till coalescence to the model boundary. When the loading rate was 0.032 mm/sec, two tensile cracks were initiated from the left and right of the hole and propagated perpendicular to the loading axis. The branching occurs in these cracks. This shows that the crack propagation is under quasi-static conditions. When the loading rate was 0.064 mm/sec, mixed tensile and shear cracks were initiated below the loading walls and branching occurred in these cracks. This shows that the crack propagation is under dynamic conditions. The loading rate increases and the tensile strength increases. Because all defects mobilized under a low loading rate and this led to decreasing the tensile strength. The experimental results for the direct tensile strengths of asphalt specimens of different ingredients were in good accordance with their corresponding results approximated by DEM software.

• 한국지반신소재학회논문집
• /
• 제9권1호
• /
• pp.47-57
• /
• 2010

기초 지반이나 댐, 사면 등을 건설할 경우, 성토재로 입자 크기가 큰 조립 재료를 많이 사용하고 있다. 따라서 이러한 조립재료의 전단 거동은 구조물의 안정성에 영향을 미치게 된다. 예를 들어, 구조물과 입자의 접촉면 혹은 입자들간의 접촉면에서 발생하는 입자 파쇄는 전체 지반의 특성을 변화시키고 따라서 구조물의 안정성에 문제를 유발할 수 있다. 본 연구에서는 입자의 파쇄 유무에 따른 전단 거동의 특성을 파악하기 위해 개별요소법(DEM, Distinct Element Model)을 기반으로 하는 수치해석 프로그램 PFC2D를 이용하여 직접전단실험을 재현하였다. 입자의 모델을 파쇄 모델과 비 파쇄 모델로 구분하여 총 4개의 모델을 모사하고 그 결과를 분석하였다. 비 파쇄 모델에는 one ball 모델과 clump 모델이, 파쇄 모델에는 cluster 모델과 Lobo-crushing 모델을 적용하였다. 입자의 구성은 Lobo-Guerrero and Vallejo(2005)가 제안한 8개 입자의 조합으로 구성하였다. 해석 결과, 내부마찰각 순서는 clump 모델 > cluster 모델 > one ball 모델 순이며, 전체를 비교해 봤을 때 원형입자모델보다 입자 결합모델이, 파쇄 모델보다 비 파쇄 모델의 내부마찰각이 크게 나타났다. 또한 기존에 제시된 Lobo-Guerrero and Vallejo(2005)의 모델은 입자 파쇄 거동을 모사하기에는 부적합하다는 결론을 얻을 수 있었다.